● `color{Brown}"Nucleus"` as a cell organelle was first described by `color{Brown}"Robert Brown"` as early as `color{violet}"1831"`.

● Later the material of the nucleus stained by the `color{violet}"basic dyes"` was given the name `color{Brown}"chromatin"` by `color{Brown}"Flemming"`.

● The `color{Brown}"interphase nucleus"` (nucleus of a cell when it is not dividing) has highly extended and elaborate `color{violet}"nucleoprotein fibres"` called `color{Brown}"chromatin"`, nuclear matrix and one or more spherical bodies called `color{Brown}"nucleoli"` (sing.: nucleolus).

● `color{violet}"Electron microscopy"` has revealed that the `color{Brown}"nuclear envelope"`, which consists of two parallel membranes with a space between `color{violet}"(10 to 50 nm)"` called the `color{Brown}"perinuclear space"`, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.

● The `color{violet}"outer membrane"` usually remains continuous with the `color{violet}"endoplasmic reticulum"` and also bears `color{violet}"ribosomes"` on it.

● At a number of places the `color{violet}"nuclear envelope"` is interrupted by `color{Brown}"minute pores"`, which are formed by the `color{violet}"fusion"` of its two membranes.

● These `color{violet}"nuclear pores"` are the `color{violet}"passages"` through which `color{violet}"movement of RNA and protein"` molecules takes place in `color{violet}"both directions"` between the nucleus and the cytoplasm.

● Normally, there is only `color{violet}"one nucleus per cell,"` variations in the number of nuclei are also `color{violet}"frequently observed"`.

● Some mature cells even `color{violet}"lack nucleus"`, e.g., `color{brown}"erythrocytes of many mammals"` and `color{brown}"sieve tube cells"` of vascular plants.

● The `color{violet}"nuclear matrix"` or the `color{Brown}"nucleoplasm"` contains `color{violet}"nucleolus and chromatin"`.

● The `color{Brown}"nucleoli"` are `color{violet}"spherical structures"` present in the `color{violet}"nucleoplasm"`.

● The content of nucleolus is `color{violet}"continuous"` with the rest of the nucleoplasm as it is `color{violet}"not a membrane bound"` structure.

● It is a site for `color{violet}"active ribosomal RNA synthesis"`.

● `color{violet}"Larger and more numerous"` nucleoli are present in cells actively carrying out `color{violet}"protein synthesi"`s.

● The `color{violet}"interphase nucleus"` has a loose and indistinct network of `color{violet}"nucleoprotein fibres"` called `color{Brown}"chromatin"`.

● But during different stages of `color{violet}"cell division"`, cells show `color{Brown}"structured chromosomes"` in place of the nucleus.

● `color{violet}"Chromatin"` contains `color{Brown}"DNA"` and some basic proteins called `color{Brown}"histones"`, some `color{violet}"non-histone proteins"` and also `color{Brown}"RNA"`.

● A single human cell has approximately `color{violet}"two metre long thread"` of DNA distributed among its `color{violet}"forty six"` (twenty three pairs) `color{violet}"chromosomes"`.

● Every chromosome essentially has a `color{violet}"primary constriction"` or the `color{Brown}"centromere"` on the sides of which `color{violet}"disc shaped structures"` called `color{Brown}"kinetochores"` are present.



● Based on the `color{Brown}"position of the centromere"`, the chromosomes can be classified into `color{Brown}"four type"`.

● `color{violet}"The Metacentric chromosome"` has `color{violet}"middle centromere"` forming `color{violet}"two equal arms"` of the chromosome.

● `color{Brown}"The sub-metacentric chromosome"` has `color{violet}"centromere nearer to one end"` of the chromosome resulting into `color{violet}"one shorter arm and one longer arm"`.

● In case of `color{Brown}"acrocentric chromosome"` the `color{violet}"centromere is situated"` `color{violet}"close to its end"` forming one extremely `color{violet}"short and one very long arm"`, whereas the `color{Brown}"telocentric chromosome"` has a `color{violet}"terminal centromere"`.

● Sometimes a few chromosomes have `color{violet}"non-staining secondary constriction"`s at a constant location.

● This gives the appearance of a `color{violet}"small fragment"` called the `color{Brown}"satellite"`.

● Many `color{violet}"membrane bound minute vesicles"` called `color{Brown}"microbodies"` that contain `color{violet}"various enzymes"`, are present in both `color{violet}"plant and animal cells"`.